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I'm going to assume that the actual blower fan doesn't slow down, but the air flow from the vents goes away until you release the accelerator pedal. This is due to a vacuum leak somewhere in the climate system. At idle or light gas pedal, the engine system draws enough vacuum to overcome the leak and maintain proper vent positions. During acceleration, engine vacuum goes very low. Normally, without a leak, the climate system can maintain vacuum and keep position of the dampers. With a leak however, the vacuum can be lost, and thus allowing airflow to change in the system. The most common leakage point to cause this is the bi-level diaphragm on the left side of the climate system. This controls the vent/defrost/floor functions of the climate system. In some cases, you may hear an air type of noise in the car until you get into the gas a bit. There is some 2-3 hours of labor roughly and I'm unsure of what parts would cost. It is a fault that you could sort of live with, so to speak.. The system for quite a while will probably work normally, with the exception of the acceleration fault. Obviously this isn't the correct way the car is supposed to operate, but some people have learned to live with it. It is definitely repairable without too much expense though.
Keep me posted, Make sure the timing belt was done (due @ 70,000 miles).
Thanks, Bill
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Thanks again, Bill